fixes #7638; awaitAny blocks if the flow vars all have been complete already

tests/parallel/twaitany.nim

@@ -0,0 +1,35 @@
+discard """
+  output: '''true'''
+"""
+
+# bug #7638
+import threadpool, os, strformat
+
+proc timer(d: int): int =
+  #echo fmt"sleeping {d}"
+  sleep(d)
+  #echo fmt"done {d}"
+  return d
+
+var durations = [1000, 2000, 3000, 4000, 5000]
+var tasks: seq[FlowVarBase] = @[]
+var results: seq[int] = @[]
+
+for i in 0 .. durations.high:
+  tasks.add spawn timer(durations[i])
+
+var index = awaitAny(tasks)
+while index != -1:
+  results.add ^cast[FlowVar[int]](tasks[index])
+  tasks.del(index)
+  #echo repr results
+  index = awaitAny(tasks)
+
+doAssert results.len == 5
+doAssert 1000 in results
+doAssert 2000 in results
+doAssert 3000 in results
+doAssert 4000 in results
+doAssert 5000 in results
+sync()
+echo "true"